1. Field of the Invention
The present invention relates to an optical head device for optically recording/reproducing information to and from an information recording medium (a so-called optical disk), and particularly to a technique for reducing unstableness of operation caused by a difference in wavelength between lights used in recording and reproduction.
2. Description of the Background Art
The characteristics such as reflectance of an information recording medium (a so-called optical disk) used to optically record/reproduce information have wavelength dependence. Accordingly it is necessary to record/reproduce information by using light at a given wavelength corresponding to the information recording medium. Optical head devices having light sources of different wavelengths are suggested, where the light emitted from each light source is applied to the information recording medium.
FIG. 10 is a schematic diagram showing the structure of a first conventional optical pickup device 201P. The optical pickup device 201P is disclosed in Japanese Patent Application Laid-Open No. 11-296893 (1999).
The optical pickup device 201P has a beam shaping prism (hereinafter referred to also as shaping prism) 31P, a light source 32P emitting light at a wavelength xcex1, a light source 33P emitting light at a wavelength xcex2 ( greater than xcex1), a prism 34P, a collimator lens 35P, a beam splitter 36P, a deflection prism 37P, an objective lens 38P, an optical disk 39P, a condenser lens 40P, a cylindrical lens 41P and a photodetector 42P.
In the optical pickup device 201P, the light at the wavelength xcex1 emitted from the light source 32P and the light at the wavelength xcex2 emitted from the light source 33P are combined by the prism 34P and then sequentially enter the collimator lens 35P and the shaping prism 31P. The collimator lens 35P converts the lights into approximately parallel rays and the shaping prism 31P shapes their beam form.
The lights exit from the shaping prism 31P at about the same angles and their optical paths are deflected by 45 degrees by the deflection prism 37P. The lights are then focused by the objective lens 38P to form very small spots on the optical disk 39P. Information is recorded/reproduced to and from the optical disk 39P through the small spots.
The lights reflected at the optical disk 39P are converted again into approximately parallel rays by the objective lens 38P, reflected at the beam splitter 36P, and condensed by the condenser lens 40P. The cylindrical lens 41P introduces astigmatism to the lights and the lights are received at the photodetector 42P. The photodetector 42P outputs an information signal and a servo signal.
Next, FIG. 11 is a diagram schematically showing the structure of a second conventional optical pickup device 202P. The optical pickup device 202P is disclosed in the above-mentioned reference. The optical pickup device 202P has a light source unit 43P in place of the light sources 32P and 33P in the above-described optical pickup device 201P. The light source unit 43P has two light sources, or two semiconductor laser chips (hereinafter also referred to as LD chips) sealed in a single can. The two LD chips are spaced at an appropriate interval on the same plane which is vertical to the optical axis of the collimator lens 35P. The LD chips respectively emit light at a wavelength xcex1 and light at a wavelength xcex2. The lights emitted from the two LD chips are converted into approximately parallel rays by the collimator lens 35P and shaped by the shaping prism 31P. After passing through the shaping prism 31P, the lights travel the same optical path as that in the optical pickup device 201P.
The conventional optical pickup devices 201P and 202P have the following problems. First, the optical pickup device 201P has the two light sources 32P and 33P prepared as separate parts and disposed separately. Therefore the device has a larger number of parts and a complex structure, and must be large in size.
On the other hand, in the optical pickup device 202P, the two LD chips (light sources) are disposed in the single can and the lights at the two wavelengths are received at a single photodetector. Therefore it can solve the above-mentioned problem. However, the two LD chips are just arranged at an appropriate interval on the same plane which is vertical to the optical axis of the collimator lens 35P. Accordingly, the wavelength dependence of the refractive index in the optical axis direction of each optical part causes offset in a servo signal corresponding to at least one of the two lights of different wavelengths, which leads to another problem that the operation becomes unstable.
Furthermore, when two LD chips are separately prepared and simply arranged in a hybrid manner using an assembling machine, the relative position of the two LD chips depends on the mechanical accuracy of the assembling machine etc. That is to say, the positions of the two optical sources are likely to shift relative to each other. Such positional shift will cause offset in the servo signal and then the lights at the two wavelengths cannot be received accurately enough.
(1) According to a first aspect of the present invention, an optical head device comprises: a light source comprising a first light emitting point for emitting a first light at a first wavelength and a second light emitting point for emitting a second light at a second wavelength in approximately the same direction as the first light, the first wavelength and the second wavelength differing from each other; a collimator lens disposed to face to the first light emitting point and the second light emitting point, for converting the first light and the second light into approximately parallel lights; and a beam shaping prism having a plane of incidence, the first light and the second light as the approximately parallel lights obliquely entering the plane of incidence, the beam shaping prism changing sectional intensity distributions of the first light and the second light to approximately circular shape; wherein the collimator lens and the beam shaping prism have refractive indexes dependent on a wavelength of incident light, and the first light emitting point and the second light emitting point are placed in different positions from each other in a direction parallel to and a direction vertical to an optical axis of the collimator lens.
(2) Preferably, according to a second aspect, in the optical head device, the first wavelength is shorter than the second wavelength and the refractive index of the collimator lens to the first wavelength is larger than the refractive index to the second wavelength, and wherein the first light emitting point is placed closer, than the second light emitting point, to the collimator lens.
(3) Preferably, according to a third aspect, in the optical head device, the first wavelength is shorter than the second wavelength and the refractive index of the beam shaping prism to the first wavelength is larger than the refractive index to the second wavelength, and wherein the first light enters the plane of incidence of the beam shaping prism at an angle of incidence which is larger than an angle of incidence at which the second light enters the plane of incidence.
(4) Preferably, according to a fourth aspect, in the optical head device, the first light emitting point and the second light emitting point are formed monolithically.
(5) Preferably, according to a fifth aspect, the optical head device further comprises a first optical system comprising the beam shaping prism, for directing the first light and the second light from the light source to a recording medium, a photodetector having a light receiving portion receiving the first light and the second light reflected at the recording medium, a sensor lens having a refractive index dependent on a wavelength of incident light, for converging the first light and the second light onto the light receiving portion, and a second optical system comprising the sensor lens, for directing the first light and the second light reflected at the recording medium to the photodetector.
(1) According to the first aspect of the invention, the first light emitting point and the second light emitting point are disposed in different positions in the direction parallel to the optical axis of the collimator lens and in the direction vertical to the optical axis of the collimator lens. It is then possible to cancel the wavelength dependence of the refractive indexes of various optical parts in the optical head device, including the collimator lens and the beam shaping prism, and to cancel the wavelength dependence of functions induced with refractive indexes, by adjusting the positions of the first light emitting point and the second light emitting point. Further, the wavelength dependence can be canceled at once both in the direction parallel to and the direction vertical to the optical axis in the optical head device. Accordingly, in an optical head device using a common optical system for the first light and the second light at different wavelengths, operations to both of the first light and the second light can be performed accurately and stably.
Furthermore, since the first light and the second light are emitted in approximately the same direction, the light source can be constructed as a single part or element. The optical head device can thus be constructed with a smaller number of parts and can be simpler in structure, as compared with a device having the first light emitting point and the second light emitting point as separate light sources. Moreover, the optical head device can be downsized.
(2) According to the second aspect, in the direction parallel to the optical axis of the collimator lens, it is possible to cancel the wavelength dependence of functions which the collimator lens etc. exert on the first light and the second light. Accordingly, by using the collimator lens in combination with a sensor lens whose refractive index to the first wavelength is larger than that to the second wavelength and which converts parallel light into converging light, the first light and the second light as parallel rays can be focused or converged onto the same position in the direction parallel to the optical axis of the sensor lens. This alleviates the problem caused if the first light and the second light are focused or converged in different positions, thus allowing the optical head device to accurately and stably operate.
(3) According to the third aspect, on the basis of the wavelength dependence of the refractive index of the beam shaping prism and the incidence angles of the first light and the second light to the beam shaping prism, it is possible to cause the first light and the second light to exit from the plane of incidence of the beam shaping prism at the same exit angle. That is to say, it is possible to cause the first light and the second light to become parallel to each other after they have passed through the plane of incidence of the beam shaping prism. This alleviates the problem caused if the first light and the second light are not parallel to each other, thus enabling the optical head device to operate accurately and stably. The incidence angles can be set as above by adjusting the positions of the first light emitting point and the second light emitting point in the direction vertical to the optical axis of the collimator lens.
(4) According to the fourth aspect, the first light emitting point and the second light emitting point are formed monolithically or as a single part. Therefore, the first light emitting point and the second light emitting point can be more precisely disposed than in a device where two light emitting points are separately prepared and simply arranged in a hybrid manner. Therefore the optical head device can operate more accurately and more stably.
(5) According to the fifth aspect, the wavelength dependence of the refractive indexes of the collimator lens, beam shaping prism and sensor leans, and the wavelength dependence of functions induced with refractive indexes, can be canceled by adjusting the positions of the first light emitting point and the second light emitting point. Accordingly the first light and the second light can both be focused or converged as spots in the same position on the light receiving portion of the photodetector, which reduces the offset of the operation or output signal of the photodetector which would be caused by the difference between the first wavelength and the second wavelength. As a result, the optical head device can operate accurately and stably.
The present invention has been made from the above-described viewpoints, and an object of the present invention is to provide an optical head device having simple structure and capable of reducing unstableness of operation caused by a difference in light wavelength.